1. Field of the Invention
This invention relates generally to the field of magnetic resonance imaging systems and more specifically to an antenna coupling circuit for a coil antenna of a magnetic resonance imaging system.
2. Description of the Related Art
Magnetic resonance imaging ("MRI"), also known as nuclear magnetic resonance ("NMR") imaging, has become a valuable tool as a safe, non-invasive means for obtaining information in the form of images of an object under examination. For example, MRI can be used as a medical diagnostic tool by providing images of the whole or selected portions of the human body without the use of X-ray photography.
MRI systems take advantage of the magnetic properties of spinning nuclei N of chemical species found in the observed object. Each of the nuclei has an internal spin axis and a magnetic pole aligned with the spin axis. The magnetic pole is a vector quantity representing the magnitude and direction of the magnetic field of the nucleus. Application of an external static magnetic field B.sub.o causes the magnetic poles to align themselves along the external magnetic field lines.
The MRI system disturbs this alignment by transmitting an electromagnetic signal to the object. The magnetic field B.sub.1 of this transmitted electromagnetic signal is circularly polarized and is perpendicular to the static magnetic field B.sub.o. This signal causes the nuclei to precess about the external static magnetic field lines. The frequency of this precession typically is in the radio frequency ("RF") range. More specifically, the precession frequency generally lies within a relatively narrow bandwidth of about 1 to 100 kHz at a center frequency of between 1 and 100 MHz.
As the nuclei precess, they radiate an electromagnetic signal having a circularly polarized rotating magnetic field. The frequency of this rotating magnetic field is generally equal to the precession frequency of the nuclei. The radiated signal is received by the MRI system to produce an image of the object under examination.
MRI systems may employ an inductive coil antenna to transmit and receive the RF signals. The antenna is electrically coupled to a low-noise amplifier of a receiving circuit which senses the voltage from the antenna. A signal processor then processes this voltage signal to produce an image of the object.
These MRI systems usually include antenna coupling circuitry for coupling the antenna coil to the receiving circuit. This coupling circuitry typically includes a parallel resonant circuit coupled to a low-noise receiving amplifier A of the receiving circuit, as shown in FIG. 1. The antenna coil is the inductor L of the resonant circuit. The capacitor C represents the total capacitance of the coupling circuit in parallel with coil L. The resistance R represents the total resistance of the coupling circuit in parallel with coil L. At the resonant frequency f.sub.o of the circuit, the reactance becomes zero and the impedance is equal to the resistance R in parallel with coil L and the input terminals of amplifier A.
The quality of the image that can be produced with such an MRI system is dependent upon the signal-to-noise ratio ("SNR") of the system. In contrast with other imaging methods such as X-ray photography, the RF signal transmitted by the object under examination is essentially free of noise. The vicinity of the MRI system is generally well shielded so that noise attributable to external sources of electromagnetic radiation are minimized or eliminated. There are, however, several sources of noise within the MRI system which may significantly degrade performance. A most important noise source is attributable to thermal noise associated with electrical losses, i.e., dissipation or resistance, in the system.
As is well known, any electrical resistor at a temperature above absolute zero will have a noise power generated within it by the random thermal motion of the charge carriers. The thermal noise power or Johnson noise is equal to kTB where k is Boltzmann's constant, T is the absolute temperature, and B is the system noise bandwidth in Hertz. This noise power is independent of the current flowing through the resistor and is also independent of the resistance value. However, the noise voltage (as opposed to the noise power) is a function of resistance since the voltage V for a given power is a function of resistance R (P=V.sup.2 /R). Because amplifier A coupled to the antenna coupling circuit is responsive to voltage, the noise voltage attributable to resistances in the system affects the SNR.
There are a number of resistances in the MRI system which contribute to the noise voltage at amplifier A of the receiving circuit, two of which generally predominate. The first resistance is attributable to the electrical conductivity of the object under examination. Since the object is electrically conductive, the presence of this object near coil L creates a resistance R.sub.o in parallel with the coil. The second resistance is attributable to resistances of the individual electrical components comprising the coupling circuit. As is well known, there is a resistance associated with any practical electrical component, including capacitive and inductive elements. This second resistance R.sub.c associated with these components creates a noise voltage at amplifier A even in the absence of a conductive object near the coil.
Other sources of noise may include resistances attributable to external coupling of coil L with external conductors (conductive objects which are not part of and are external to the antenna coil and the coupling and receiving circuits, such as the frame of the MRI system), and coupling of the object under examination with such external conductors. However, these noise sources are not discussed further since they may be controlled or eliminated by appropriate design considerations and are not directly relevant to the present invention. In addition, amplifier A itself has resistances and associated noise within it, which are not relevant here but will be discussed below. Thus, antenna coupling circuits essentially include two resistances, R.sub.o and R.sub.c, in parallel with inductor L (the antenna coil). These parallel resistances may equivalently be considered to represent the total resistance R in parallel with inductor L, as shown in FIG. 1.
The sensitivity of the receiving system can be expressed quantitatively in terms of the circuit quality factor or circuit "Q" as follows: EQU Q=2.pi.E.sub.s /E.sub.d ( 1)
where E.sub.s is the energy stored in the resonant circuit during one cycle of a sinusoidal oscillation and E.sub.d is the energy dissipated during the one cycle of that sinusoidal oscillation. It can be shown that the SNR of the MRI system, assuming resistances R.sub.o and R.sub.c are the only noise sources, is proportional to the square root of the circuit Q: EQU SNR.alpha.Q.sup.0.5 ( 2)
The circuit Q may also be expressed in terms of the inductive reactance X.sub.L of coil L and resistance R in parallel with coil L as follows: EQU Q=R/X.sub.L ( 3)
The inductive reactance X.sub.L of the circuit is defined by the following equation: EQU X.sub.L =2.pi.f.sub.o l (4)
where f.sub.o is the resonant frequency of the circuit and l is the inductance of coil L. At the resonant frequency f.sub.o, the inductance l of coil L and the capacitance C of the circuit are related by the following mathematical relationship: EQU (2.pi.f.sub.o).sup.2 =1/(lC) (5)
Thus, as shown in equation 3 above, obtaining a high circuit Q for the parallel resonant circuit of FIG. 1 requires parallel resistance R to be large or inductive reactance X.sub.L to be small, or both. For a given antenna design and therefore a given inductive reactance X.sub.L, improvement in system performance may be achieved by making the value of resistance R in parallel with coil L as large as possible given practical constraints placed on the system.
Moreover, an efficient design requires that resistance R.sub.c of the circuit components should be much larger than resistance R.sub.o of coil L from the object under observation. This corresponds to a relatively high value for the circuit Q of the resonant circuit itself, without the presence of an object in the vicinity of coil L. Of course, coil L itself should have low series resistance or, equivalently, high parallel resistance. Thus, a coupling circuit having relatively greater resistance R.sub.c of its components in parallel with coil L is needed.
Practical antenna coupling circuits generally include a number of electrical components in addition to those shown in FIG. 1. For example, the capacitance C of FIG. 1, which represents the total capacitance of the resonant circuit, generally includes a tuning capacitor and one or more direct current ("DC") blocking capacitors. The resistance R in the circuit of FIG. 1, which represents the total resistance of the circuit in parallel with coil L, is equal to the resistance R.sub.o introduced by the presence of the object near coil L and the resistance R.sub.c of the coupling circuit components, since the presence of other losses has been assumed away.
The resistance R.sub.o in parallel with the antenna coil attributable to the presence of the object to be examined is not generally amenable to design variation without interfering with the RF signals used by the MRI system. However, the resistance R.sub.c associated with the coupling circuit components is generally amenable to design variations which increase the value of the total resistance R in parallel with coil L and thereby increase the circuit Q.
It is possible to increase the resistance R.sub.c of the components in parallel with the coil, for example, by increasing the resistance of parallel resistors in the circuit. However, increasing the resistance R.sub.c of the components also has the undesirable effect of increasing the actual resistance in the coupling circuit, which results in increased power consumption and noise voltage.
Accordingly, it is an intent of the invention to provide an antenna coupling circuit which has an enhanced SNR and a correspondingly enhanced circuit Q while avoiding an increase in actual resistance and noise of the circuit.
A further intent of the invention is to provide an antenna coupling circuit with high apparent resistance in parallel with the antenna and low actual resistance in the circuit.
It is still further an intent of the invention to provide an antenna coupling circuit which, in addition to satisfying the intentions set forth above, balances the antenna coil with respect to a ground.
Additional intentions and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The intentions and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.